Sedimentation event sensor: New ocean instrument for in situ imaging and fluorometry of sinking particulate matter

Abstract

Measuring the quantity and composition of sinking particulate matter is key to understanding biogeochemical processes in the ocean. This has been done in the past with sequencing sediment traps, which collect and store particulate matter in sample bottles for subsequent laboratory analysis. Having a limited number of bottles, these traditional traps force the user to choose between finer time resolution and longer deployment duration. We have built a new sediment trap that analyzes the collected material in situ, eliminating the need to preserve and store the samples. This new instrument, the Sedimentation Event Sensor (SES), captures macro images of the sample with front and back lighting, and takes fluorometric measurements in two bands as proxies for the presence of chlorophyll a and accessory pigments. The SES can process 6200 samples during a single deployment, which is the equivalent of 15 samples per day for more than a year. Here, we describe the design of the SES and present the results of its first three deployments at 3910 m depth. Images and fluorometry data revealed high variability of sinking particulate matter composition on the order of hours. Sedimentation patterns detected by the SES largely agreed with mass flux patterns measured from traditional traps deployed concurrently nearby. Given the functional differences, the SES is best used to complement rather than replace traditional sediment traps.